Patterning layers of photoresist using optical lithography to provide desired circuits or other structures on a semiconductor wafer is known. The photoresist material is imaged using a mask having the desired pattern after which a portion of the resist is removed to expose the underlying layer of the semiconductor wafer. Additional processing may then be performed, such as depositing materials in the exposed areas, etc.
To increase speed and performance of integrated circuits, it is desirable to continually decrease the dimensions of structures, such as traces, etc. that are placed in the exposed areas on the wafer. One factor that limits the dimensions in optical lithography is reflection of the light used to expose the photoresist. The reflected list adversely affects the control over dimensions by, for example, exposing photoresist outside of the desired areas which can lead to undercutting. Another undesirable effect of reflections from the underlying surface is the creation of standing waves if the illuminating light is monochromatic. The standing waves can vary the development of the resist material along the edges of the pattern, thereby decreasing the image resolution.
The additional exposed photoresist will result in variations in the desired dimensions of the areas exposed in the photoresist material. As a result, the areas exposed in the photoresist will also vary. Although those variations can be accounted for in the design of the patterns, they do limit the minimum dimensions that can be accurately patterned.
Reflectance problems are particularly troublesome when the layer underneath the photoresist is aluminum. Aluminum is widely used in the manufacture of integrated circuits because of its low melting point, high conductivity and low cost. It is, however, highly reflective which enhances the reflectivity problems discussed above.
One attempt at reducing reflectance of aluminum beneath a layer of photoresist to enhance resolution in optical lithography involves depositing an anti-reflective coating (ARC) on the aluminum, beneath the photoresist, to absorb light reaching the aluminum to prevent it from exposing unwanted areas of photoresist. One example of an anti-reflective coating is amorphous silicon as described in U.S. Pat. No. 5,441,616 to Nanda, et al. Another example is a sputtered layer of TiN as described in U.S. Pat. No. 5,427,666. When using TiN, however, reflectance is typically reduced to about 20% of the incident light and is only that effective over relatively narrow range of wavelengths.